Internal damage processes in low alloy chromium–molybdenum steels during high-temperature creep service

Results are presented of investigations into structure of low alloy Cr–Mo steels exhibiting internal damage after long-term creep service. It was demonstrated that intercrystalline cavitation cracks were the dominant factor in service damage of power station boiler components operating in creep regimes. Consecutive stages in development of internal damage involving intercrystalline cavitation cracking were discussed and illustrated with micrographs. The results seem to indicate that nucleation of creep cavities in materials under consideration is related to grain boundary slip. Evidence confirming the shear mechanism proposed by Sklenička and Saxl for cavity coalescence was obtained. Occurrence of intercrystalline crevice cracking was demonstrated. Micrographs were used to illustrate wedge crevice nucleation modes on triple junctions depending on the direction of slip according to Chang and Grant. A classification of internal damage in relation to life exhaustion was proposed for materials under consideration. A method used in industrial practice for evaluation and qualification of creep-damaged materials was presented.